An electronically controlled internal combustion engine which is equipped with various devices such as an electronic fuel injection system and/or an electronic ignition timing control system, requires a measuring apparatus which provides an electric signal indicative of the rotational information such as the rotational speed of the crankshaft of the engine or the advance angle of the ignition timing.
In one such apparatus a circular disk or body is fixedly connected to the crankshaft of an internal combustion engine so that the disk rotates in synchronization with the crankshaft. The circular disk has a plurality of gear teeth which have an equal circular pitch, along the peripheral surface of the disk. At least one marker, such as a projection, which is utilized for detecting the reference point or position of the circular disk, is mounted on the circular disk at a different position with respect to the gear teeth. Two pick-ups, which are of the electro-magnetic type, are respectively disposed on a fixed stationary member and are respectively arranged adjacent the peripheral surface and the circular track of the marker. These two pick-ups respectively emit a train of output impulse signals when the circular disk rotates since the magnetic resistance between the first pick-up and the gear teeth and the magnetic resistance between the second pick-up and the marker vary as the disk rotates. A first signal produced by the first pick-up provides calibration pulses via which angular measurement can be made while a second signal produced by the second pick-up is used to indicate a reference time or position which corresponds to the reference position from which an angle can be measured using the calibration pulses.
In case of utilizing such an apparatus, there are two methods for increasing the accuracy of the measurement of the advance angle. The first method is to increase the number of the teeth, i.e. the calibrating members, and the second method is to employ more than two pickups which are aligned in a direction of the rotation of the circular disk, for producing, via suitable circuitry, a signal indicative of the phase difference between the output signals thereof. In the former case, precision machining for producing the large number of gear teeth is required and thus the first method is not suited to mass-production. In the latter case, at least three pick-ups are necessary, viz. two for the teeth and one for the marker. The increase of the number of pick-ups causes a cost increase since each pick-up is relatively expensive. Moreover, if more than two pick-ups are disposed adjacent the peripheral surface of the disk, it is necessary to arrange these two pick-ups within a circular pitch of the teeth. Furthermore, an adjustment of the space or gap between the pick-ups and the peripheral surface of the disk is troublesome and time consuming.
In another type of an apparatus for measuring the advance angle, only one pick up is employed for sensing both of the gear teeth and the marker. In this type of apparatus, the marker is disposed between two specific teeth, so that the single pick-up, which is arranged adjacent the peripheral surface of the circular disk, produces a train of pulse signals as the disk rotates. In order to distinguish a signal indicative of the reference position or angle from the other calibration pulse signals, the circular pitch between the two specific teeth is different from those of the other teeth which are equidistantly spaced or angularly displaced whereby the phases of the two signals are different from each other. Consequently, the output signal of the pick-up will be divided, by means of the phase difference, into two signals which respectively indicate the reference position and the calibration signals.
Apart from the above described conventional apparatus, some technicians of Nissan Motor Company Limited have proposed a new apparatus for measuring rotational angle of a rotary member in a Japanese Patent Application No. (52-103399). According to the new apparatus, the calibration gear teeth and a marker are aligned on the same periphery of a rotary disk so that a single pick-up produces both of calibration pulses and a marker pulse per revolution of the rotary disk. The marker is actually a teeth-omitted portion, i.e. the calibration gear teeth are equidistantly arranged along the periphery of the disk except the marker portion. In other words, the circular pitch of teeth adjacent to the marker portion is different from other pitches of the calibration gear teeth. In the above mentioned apparatus, if it is desired to increase the accuracy of measurement, the number of the pick-ups may be increased.
Although the apparatus for measuring rotational angle of a rotary member measures the rotational angle such as an advance angle of ignition timing, correctly, when it is intended to use the same disk and pick-up(s) for the measurement of the rotational speed of the rotary member, the rotational speed is erroneously detected due to the marker, i.e. the teeth-omitted portion. Since the basic idea of measurement of the rotational speed is to count the number of pulses produced in response to the passage of the gear teeth which are equidistantly arranged, for a predetermined period of time, if some of the gear teeth have different circular pitch with respect to other gear teeth, the number of the pulses counted per unit time does not correctly indicate the actual rotational speed.
It will be understood that it would be convenient if the same parts such as the rotary disk, pick-up(s) and a portion of an electrical circuit are used for both of a rotational angle measuring system and a rotational speed measuring system. However, as described hereinabove, when some of the gear teeth are not equidistantly arranged, the pulse number per unit time does not indicate the actual rotational speed.